This article presents experimental studies of state of charge (SOC) monitoring of vanadium species in sulfuric acid. An IR sensor for run-time measurement of SOC of electrolytes of vanadium RedOx flow battery was constructed. The presented method allows the independent measurement of the SOC of catholyte as well as anolyte using identical sensors with an accuracy better than +/- 1%. (c) 2013 Elsevier B.V. All rights reserved.